A neutron detector is an elemental technology underlying neutron utilization technologies. A neutron detector of higher performance is desired with the development of neutron utilization technologies in security fields such as cargo inspection, academic and research fields such as structure analysis by neutron diffraction, nondestructive inspection fields, or medical fields such as boron neutron capture therapy.
Neutron detection efficiency and the ability to discriminate between a neutron and a gamma ray (may hereinafter be referred to as neutron-gamma or n/γ discrimination ability) are named as important characteristics required of a neutron detector. The neutron detection efficiency is the ratio of the number of neutrons counted by the detector to the number of neutrons entering the detector. When the detection efficiency is low, the absolute number of the neutrons measured is small and the accuracy of measurement lowers.
A γ-ray exists in the natural world as natural radiation, and also occurs when a neutron hits a constituent member of a detection system for detecting a neutron, or when a neutron collides with an object to be inspected. If the n/γ discrimination ability is low, therefore, a γ-ray is counted as a neutron, so that the accuracy of neutron counting declines.
The present inventors have proposed, as a scintillator excellent in the above-mentioned neutron detection efficiency and n/γ discrimination ability, a neutron scintillator comprising a eutectic having layered lithium fluoride crystals and layered calcium fluoride crystals stacked in a specific structure (see Patent Document 1).
A eutectic comprising lithium fluoride and an inorganic fluorescent material and having a characteristic texture structure can be used as a neutron scintillator.
In the eutectic described in Patent Document 1, the inorganic fluorescent material is calcium fluoride crystals, and the form of the texture structure is a layered structure. In such a eutectic, Li-6 isotope in the LiF causes a capture reaction with a neutron, and a secondary particle resulting from the reaction imparts its energy to the inorganic fluorescent material to cause light emission, thereby acting as a neutron scintillator.
A neutron scintillator comprising the above eutectic is advantageous in that it has a high detection efficiency for neutrons and excels in n/γ discrimination ability. However, it has been difficult to obtain a eutectic having a uniform texture structure, and the resulting scintillator has posed the problem that its characteristics tend to be nonuniform. Concretely, there have been problems such that in the above exemplified layered structure, the thickness of the layers is not uniform, the layered structure itself is not uniform, and the orientation of the layers is different.
To solve the above-mentioned problems, Patent Document 1 proposes that a eutectic having a uniform texture structure be obtained by a unidirectional solidification process using the Bridgman method, the temperature gradient solidification method, the Czochralski method, or the micro-pulling-down method, and the resulting eutectic be used in a neutron scintillator.
Even when such a unidirectional solidification process is used, however, it is quite difficult to obtain a eutectic of a homogeneous texture structure efficiently and reproducibly. Furthermore, an apparatus for use in this method has the problems of being expensive and requiring a lengthy time for production, and leaves room for improvement in the reduction of a manufacturing cost.
Besides, the eutectic has the following problems: Lithium fluoride and calcium fluoride, which are the constituent components of the eutectic, have different thermal expansion coefficients. Thus, cracking due to distortion is apt to occur in a cooling process during production. This makes it difficult to produce a large-sized scintillator.